Expandable polyolefin particles

ABSTRACT

The invention relates to expandable polyolefin beads which comprise blowing agent and have been coated with from 0.01 to 3% by weight of a salt and/or ester of a long-chain fatty acid, preferably with calcium stearate, to prevent caking during foaming.

The invention relates to an expandable polyolefin bead material whichhas been coated with a means of preventing caking during foaming.

Increasing quantities of polyolefin foam beads are used to produce foammoldings in automotive construction, for packaging, and in the leisuresector. Expanded polypropylene foam beads (EPP) are producedindustrially by impregnating polypropylene pellets with a volatileblowing agent in aqueous suspension under superatmospheric pressure andthen reducing the pressure, whereupon the impregnated beads foam.Blowing agents used in industry are butane, dichlorodifluoromethane, andCO₂. Since these blowing agents diffuse out of the polypropylene againrelatively quickly, polypropylene beads comprising blowing agent andproduced in this way are considered not to be capable of storage.

German patent applications 199 50 420 and 100 03 021 have for the firsttime described expandable polyolefin beads comprising blowing agent andhaving a bulk density of more than 400 g/l which are capable of storageover a prolonged period without escape of any substantial amount ofblowing agent. They can therefore be foamed to bulk densities of lessthan 200 g/l using superheated steam at superatmospheric pressure. DE199 50 420 produces the beads by using an organic blowing agent,preferably pentane, to impregnate polyolefin pellets in aqueoussuspension in a pressure vessel, at an elevated temperature, and coolingthe mixture to below 100° C. prior to pressure reduction. DE 100 03 021mixes polypropylene pellets with the organic blowing agent and anucleating agent in an extruder, with extrusion and pelletization underconditions of temperature and pressure such as to avoid foaming.

It has now been found that the expandable polyolefin beads tend to cakeand clump during foaming, in particular if operations are carried out atrelatively high temperatures, i.e. at a high steam pressure, in order toachieve bulk densities below 100 g/l. Since the production of theexpandable beads by impregnation with blowing agent in aqueoussuspension always requires the addition of relatively large amounts ofmetal carbonates or metal phosphates as dispersing agents to preventcaking of the polyolefin pellets used within the suspension, an obviousmethod would have been to avoid removing these dispersing agents and toallow them to remain on the resultant expandable beads. However, metalcarbonates and metal phosphates would impair the fusion of the foambeads obtained after expansion, and they would therefore have to beremoved from the foam beads by complicated acid washing processes,producing wastewater in which salts are present.

It is an object of the present invention, therefore, to provideexpandable polyolefin particles which do not cake during foaming andwhich can be fused without difficulty to give foam moldings.

We have found that this object is achieved by coating the expandablepolyolefin beads with from 0.01 to 3% by weight of a salt and/or esterof a long-chain fatty acid. This coating may remain on the foam beadsafter expansion, since it causes no, or only very little, impairment ofthe fusion process to give foams.

DE-C 19541725 describes expandable styrene polymers which have beencoated with a mixture of glycerol tristearate and a hydrophobic silicateto reduce water absorption.

For the purposes of the present invention, polyolefins are

-   -   a) homopolypropylene,    -   b) random copolymers of propylene with from 0.1 to 15% by        weight, preferably from 0.5 to 12% by weight, of ethylene and/or        a C₄-C₁₀ α-olefin, preferably a copolymer of propylene with from        0.5 to 6% by weight of ethylene or with from 0.5 to 15% by        weight of 1-butene, or a terpolymer made from propylene, from        0.5 to 6% by weight of ethylene, and from 0.5 to 6% by weight of        1-butene, or    -   c) a mixture of a) or b) with from 0.1 to 75% by weight,        preferably from 3 to 50% by weight, of a polyolefin elastomer,        e.g. an ethylene-propylene block copolymer having from 30 to 70%        by weight of propylene, or    -   d) polyethylene (LLDPE, LDPE, MDPE, HDPE) or    -   e) a mixture of the polyolefins mentioned under a) to d) (or        where appropriate with addition of compatibilizers).

The crystalline melting point (DSC maximum) of the polyolefins listedunder a) to e) is generally from 90 to 170° C., their enthalpy offusion, determined by DSC, is preferably from 20 to 300 J/g, and themelt flow index MFI (230° C., 2.16 kp for propylene polymers and 190°C., 2.16 kp for ethylene polymers) is preferably from 0.1 to 100 g/10min to DIN 53 735.

Preferred polyolefins are homo- or copolymers of propylene having up to15% by weight of ethylene and/or 1-butene, particularly preferablypropylene-ethylene copolymers having from 1 to 5% by weight of ethylene.They have a melting point of from 130 to 160° C. and a density (at roomtemperature) of about 900 g/l.

The olefin polymer may have been blended with up to 50% of its weight ofa thermoplastic of a different type and having a glass transitiontemperature (point of inflection in DSC curve) below 180° C. Examples ofsuitable thermoplastics are polyamides in amounts of from 5 to 40% byweight. Conventional compatibilizers, e.g. block copolymers, such asExxelor P 1015 (EXXON), may be added to this mixture.

It has been found that the invention may also be worked withoutadmixture of any thermoplastic of a different type. This is preferredinsofar as the presence of a foreign thermoplastic impairs therecyclability of the polyolefin and of the foam produced therefrom.Elastomeric ethylene-propylene copolymers which may be added forplasticization are not regarded for the purposes of the presentinvention as being of a different type.

The polyolefin may comprise the usual additives, such as antioxidants,stabilizers, flame retardants, waxes, nucleating agents, fillers,pigments, and dyes.

The starting material for preparing the novel expandable polyolefinbeads is polyolefin pellets, which preferably have average diameters offrom 0.2 to 10 mm, in particular from 0.5 to 5 mm. These mostlycylindrical or round mini pellets are prepared by extruding thepolyolefin, where appropriate, together with the thermoplastic to beadmixed and with other additives, and, where appropriate, cooling andpelletizing.

The mini pellets should preferably comprise from 0.001 to 10% by weightof a nucleating agent, preferably from 0.1 to 5% by weight, and inparticular from 0.5 to 3% by weight. Examples of those suitable are talcand/or waxes, and also carbon black, graphite, and fumed silicas. Theybring about the production of a small-cell foam, and in a good manycases foaming is impossible without them.

In the preferred process for preparing the expandable beads, pellets aredispersed in a suspension medium in a stirred reactor. The preferredsuspension medium is water. Suspending agents have to be added here toensure uniform distribution of the mini pellets in the suspensionmedium. Suitable suspending agents are water-insoluble inorganicstabilizers, such as tricalcium phosphate, magnesium pyrophosphate,metal carbonates, and also polyvinyl alcohol and surfactants, such assodium dodecylarylsulfonate. Amounts usually used of these are from 0.05to 10% by weight. The addition of suspension stabilizers may bedispensed with if, as in WO-A 99/10419, the density of the suspensionmedium is lower than that of the suspended pellets. This is the case,for example, if the suspension medium is ethanol or a mixture of ethanolwith up to 50% of water.

The boiling point of the blowing agent should be from −5 to 150° C., inparticular from 25 to 125° C. The blowing agent is preferably an alkane,an alkanol, a ketone, an ether, or an ester. Particular preference isgiven to pentanes and hexanes, in particular sec-pentane,3,3-dimethyl-2-butanone, and 4-methyl-2-pentanone. It is also possibleto use blowing agent mixtures. The blowing agent is preferablyhalogen-free. However, blowing agent mixtures which comprise smallamounts, preferably less than 10% by weight, in particular less than 5%by weight, of a halogen-containing blowing agent, e.g. methylenechloride or fluorohydrocarbons, should not be excluded.

The amount of the blowing agent used is preferably from 2 to 50% byweight, in particular from 5 to 30% by weight, based on the pellets. Theblowing agent may be added prior to, during, or after the heating of thereactor contents. It may be introduced all at once or in portions.

During the impregnation process the temperature should be in thevicinity of the softening point of the polyolefin. It may be above themelting point (crystalline melting point) by from 40° C. to 25° C., butshould preferably be below the melting point. In the case ofpolypropylene, preferred impregnating temperatures are from 120° C. to160° C.

Depending on the amount and nature of the blowing agent, and also on thetemperature, the pressure which becomes established in the reactor isgenerally above 2 bar but not above 40 bar.

The impregnation times are generally from 0.5 to 10 hours. Prior topressure reduction and removal from the stirred reactor, the suspensionis cooled below 100° C., preferably to 10-50° C., by, for example,passing cooling water through the reactor jacket. The low-solubilitysuspending agent is then advantageously dissolved by adding an acid.Once the pressure has been reduced and the batch discharged from thereactor, the beads comprising blowing agent are isolated from thesuspension medium and washed.

In the alternate process for preparing the expandable beads, 100 partsby weight of polyolefin pellets are introduced to an extruder togetherwith from 3 to 30 parts by weight of a volatile organic blowing agentand from 0.01 to 8, preferably from 0.1 to 5, parts by weight of afine-particle nucleating agent, and also, where appropriate, otherconventional additives, and mixed in the extruder at a temperature atwhich the mixture comprising blowing agent is molten, preferably from160 to 220° C.

Examples of suitable nucleating agents are talc, polyolefin waxes,graphite powder, fumed silicas, citric esters, and also unmodified ormodified bentonites. Other additives which may be used are antioxidants,stabilizers, flame retardants, fillers, and pigments. From 3 to 30,preferably from 5 to 25, parts by weight of the volatile organic blowingagent are metered in with the aid of a pump at a location along theextruder downstream. It is also possible for the nucleating agent and,where appropriate, other additives to be added together with the blowingagent at this point rather than upstream, and in this case it isadvantageous for the additives to be in suspension or solution in theblowing agent.

According to the invention, to prevent foaming of the melt duringdischarge from the extruder, the extrusion and pelletization of theextrudate is undertaken under conditions of temperature and pressureunder which practically no foaming of the pellets takes place. Theseconditions may vary depending on the nature of the olefin polymer, ofthe additives, and in particular the nature and amount of the blowingagent used. Ideal conditions may readily be determined by exploratoryexperiments.

A useful industrial method is underwater pelletization in a water bathwhich has a temperature below 100° C. and is under a pressure of morethan 2 bar. The temperature must not be too low, otherwise the melt setson the die plate, and it must not be too high, otherwise the meltexpands. Increasing the boiling point of the blowing agent or reducingthe amount of the blowing agent permits higher water temperatures andlower pressures to be used. In the case of the particularly preferredblowing agent sec-pentane, the ideal temperature of the water bath isfrom 30 to 60° C. and the water pressure is from 8 to 12 bar.

According to the invention, the expandable beads are coated with from0.01 to 3%, preferably from 0.1 to 1.0%, based on their weight, of ananticaking agent. This process may take place in conventional mixingassemblies, e.g. in paddle mixers. Suitable anticaking agents are metalsoaps, i.e. metal salts of long-chain, unbranched or branched, aliphaticor cycloaliphatic carboxylic acids (see “Metallic Soaps” by Szczepanek +Könen, Ullmann's Encyclopedia, 6th Edn., Wiley VCH), preference beinggiven to aliphatic C₁₆-C₃₆ monocarboxylates of bi- or trivalent metals.Particular preference is given to calcium stearate. Other suitablematerials are waxes and fats, i.e. ester of the long-chain carboxylicacids mentioned, preferably glycerol esters, in particular glycerolstearate. It is also possible to use mixtures of the salts and estersmentioned. Other suitable materials are amides of long-chain fattyacids. Hydrophobic anticaking agents have been found to be particularlysuitable.

Besides the anticaking agents of the invention, the coating preferablyalso comprises from 0.01 to 0.5% by weight of antiblocking agents and/orfrom 0.01 to 0.3% by weight of antistats, in each case based on thebeads. The antiblocking agents prevent the pulverulent anticaking agentsfrom blocking. Examples of those suitable are fine-particle silica,talc, and bentonite. The antistats prevent the anticaking agents frombecoming electrostatically charged and adhering to the wall or to thestirrer of the mixing assembly. Examples of those suitable arealkylsulfonates, alkyl sulfates, and alkyl phosphates, fatty alcoholethoxylates, and quaternary ammonium compounds.

The beads comprising blowing agent may be foamed by conventional methodsusing hot air or steam in pressure prefoamers. In the case of foamingwith steam, depending on the type of blowing agent and polymer matrix,and on the desired bulk density, the steam pressures used are from 2 to4.5 bar, and the foaming times vary from 3 to 30 sec, and thetemperature during foaming should be above 100° C., in the case ofpolypropylene in particular from 130 to 160° C. In the case of a singlefoaming procedure, the bulk densities achieved are below 200 g/l. It maybe appropriate for technical or economic reasons to foam two or moretimes in order to achieve low bulk density.

The resultant foam beads may be used to produce foam moldings by knownmethods.

The parts and percentages mentioned in the examples are based on weight.

EXAMPLES

A. Production of Polypropylene Pellets Comprising Blowing Agent

Starting Materials:

PP 1: Novolen 3200 MC; polypropylene from Targor GmbH PP 2: Novolen3300, polypropylene from Targor GmbH Wax: Luwax AF31; polyethylene (Mn3000) from BASF AG Talc: Grade HF 325 Carbon black: Elftex 570 (Cabott)Stab 1: Ultranox 626 (General Electric) Stab 2: Lowinox TBM 6 (GreatLakes Chem.) GTS: Glycerol tristearate GMS: Glycerol monostearateAntiblock: Aerosil R 972; Degussa AG Antistat: Armostat 3002; AkzoChemie GmbH ZnS: Zinc stearate MgS: Magnesium stearate CaS: Calciumstearate AIS: Aluminum distearate StS: Stearyl stearate NaS: SodiumstearateProduction of Pellets:

Pellets G 1

100 parts of PP 1 were mixed with 1 part of talc and 0.5 part of wax andprocessed through a twin-screw extruder to give pellets with l:d=3:1 andweight 1.3 mg.

Pellets G 2

100 parts of PP 2 were mixed with 2 parts of carbon black, 1 part oftalc, 0.5 part of wax, and 0.05 part of each of Stab 1 and Stab 2 andprocessed to give pellets with l:d=2.1:1 and weight 1.3 mg.

Amounts of Materials for Impregnation:

Pellets: 296.1 kg Water: 658.1 kg sec-pentane 62.2 kg Calcium carbonate:13.3 kg (Calcilit lG; Alfa) Lutensol AO 3109: 53.3 g (BASF AG)Impregnation:

The starting materials were charged to a 1.36 m³ stirred vessel withcross-blade agitator and baffles. The mixture was brought toimpregnation temperature (140° C. for pellets 1 and 133° C. for pellets2) within a period of 2.5 hours and held at that temperature for 30 min,followed by cooling to room temperature and dissolution of the calciumcarbonate using 18.3 liters of 65% strength nitric acid. The impregnatedpellets were washed and centrifuged off from the water, and dried withthe aid of a pneumatic drier. A commercially available antistat wasapplied immediately prior to the drying process in order to prevent anyelectrostatic charging. The blowing agent content was 11.0% in thepellets 1 and 10.8% in the pellets 2. The DSC had a double peak withinthe melting range, the high temperature peak being at 166.2° C. for thepellets 1 and at 160.8° C. for the pellets 2.

B. Preparation of Coating Compositions

The percentages given in Table 1 of the components of each of thecoating compositions were mixed very intimately with one another for anumber of minutes. The resultant coating compositions (B1-B10) werepulverulent with good flow properties.

TABLE 1 B1 B2 B3 B4 B5 B6 B7 B8 B9 B10 ZnS 46 MgS 46 AlS 46 CaS 46 46 77NaS 77 StS 77 GMS 40 77 GTS 40 40 40 40 77 Antiblock 10 10 10 10 10 1616 16 16 16 Antistat 4 4 4 4 4 7 7 7 7 7C. Coating

The coating compositions were mixed intimately in a paddle mixer withthe expandable pellets described under A, and thus applied to thepellets. The amounts given in Table 2 are parts per 100 parts of PPpellets. The mixing time was from 2-3 min.

D. Foaming and Mechanical Properties

A steam-operated pressure prefoamer as usually used in EPS processingwas employed for the foaming process. The steam pressures were increasedstepwise, and the steam application time was in each case 12 sec. Afterthe foam beads had been dried screening was used to determine the extentof caking and the bulk density. Table 2 gives the maximum steampressures at which the extent of caking remained below 1%. Tensilestrength to DIN 53571 was determined for some of the specimens, as ameasure of fusion quality.

The higher the maximum steam pressure which can be used, the lower thebulk density which can be achieved. The lowest bulk densities weretherefore obtained using calcium stearate (Examples 6 to 8). Sincedifferent grades of polypropylene were used to reduce the pellets 1 and2, there are differences in the achievable maximum steam pressures andthe mechanical qualities of the moldings.

TABLE 2 Ex. Ex. Ex. Ex. Ex. Ex. Ex. Ex. Ex. Ex. Ex. Ex. Ex. Ex. Ex. 1 23 4 5 6 7 8 9 10 11 12 13 14 15 Coating Pellets G1 G1 G1 G1 G1 G1 G1 G1G2 G2 G2 G2 G2 G2 G2 Coating none B1 B1 B2 B3 B4 B4 B8 B4 B8 B5 B6 B7 B9B10 Amount [Pts] — 0.5 0.7 0.5 0.5 0.5 0.7 0.3 0.7 0.2 0.7 0.3 0.3 0.30.3 Foaming* Steam pressure 2.3** 2.4 2.6 3.0 2.9 3.4 4.0 4.0 3.5 3.23.3 2.5 2.1 2.5 2.1 [bar] Bulk density 222 182 121 82 90 65 53 53 54 6552 72 78 107 91 [g/l] Mechanical properties Molding densi- 77 79 58 ty[g/l] Tensile 915 816 815 strength [kPa] *the experiments listed arethose with the maximum steam pressure where the extent of cakingremained below 1%; **in this case even at 2.3 bar the extent of cakingwas much greater than 1%

1. An expandable homopolypropylene or a random copolymer of propylenewith from 0.1 to 15% by weight of ethylene or a C₄ to C₁₀ olefin beadmaterial with a bulk density of more than 400 g/l, which comprises from1 to 40% by weight of a volatile organic blowing agent, and which has acoating material, of a salt and/or ester and/or amide of a long-chaincarboxylic acid, as an anticaking agent.
 2. A bead material as claimedin claim 1, wherein the anticaking agent is a salt or an ester of analiphatic monocarboxylic acid having from 16 to 36 carbon atoms,preferably of stearic acid.
 3. A bead material as claimed in claim 2,wherein the anticaking agent is a salt of a bi- or trivalent metal,preferably calcium stearate.
 4. A bead material as claimed in claim 2,wherein the anticaking agent is a glycerol ester, preferably glyceroltristearate.
 5. A bead material as claimed in claim 1, wherein theanticaking agent is hydrophobic.
 6. A bead material as claimed in claim1, wherein the coating also comprises from 0.01 to 0.5% by weight, basedon the bead material, of an antiblocking agent.
 7. A bead material asclaimed in claim 1, wherein the coating also comprises from 0.01 to 0.3%by weight, based on the bead material, of an antistat.
 8. An expandablepolyolefin bead material as claimed in claim 1, wherein the polyolefinis a propylene polymer, preferably a copolymer of propylene with up to15% by weight of ethylene and/or 1-butene.
 9. A process for the foamingof expandable polyolefin bead material with a bulk density of more than500 g/l, using hot air or steam, to give foam beads with a bulk densityof not more than 200 g/l, which comprises using expandable polyolefinbead material with a coating comprising from 0.01 to 3% by weight, basedon the bead material, of a salt, ester, or amide of an aliphatic C₁₆-C₃₆monocarboxylic acid.